How To Convert Imported Identical Meshes to Instances (Duplicate Linked)?
As long as the origins of the various objects are in correct relation to each object, there is a simple way to do this. Select all objects that are supposed to share the same mesh data, then - in the 3D View - hit Ctrl + L to open the link menu. From there, choose Object Data:



This will re-use the mesh data from the last selected object on all other instances. You can tell that they are instances in the mesh data tab of the outliner. The user count (number next to the data name) should have appeared and increased to reflect the number of meshes that re-use the data: